This invention relates to a laser Doppler anemometer for measuring transient velocity distributions of a flowing fluid, and, more particularly, to a multiple measuring-control volume anemometer in which the velocity may be measured at a number of points at one time.
The use of laser Doppler anemometers to measure velocities have been in use for some time. In the basic anemometer a beam is split and then focussed to intersect at a point (the measuring-control volume) where the velocity is measured by detecting the Doppler frequency shift in the scattered light. However, if it were desired to measure the velocity profile in a fluid, the measuring control volume would have to be shifted to different points in the moving fluid. This would not normally be adequate for measuring transient flows, because the time required to complete a set of measurements consisting of a number of local velocities to define the profile could not be made before the instantaneous velocity is significantly changed.
In order to measure the velocity profile within a period of relatively much shorter than the momentary change, multiple control volume anemometers, such as the one described in U.S. Pat. No. 3,830,568, which issued on Aug. 20, 1974 to John B. Allen, were developed. This instrument provides a device of determining the velocity profile very close to the actual case. However, the multiple measuring control volumes have the same geometry and will generate the same laser Doppler frequencies when particles with the same velocity and direction pass through any one of the measuring control volume. The instrument, therefore, requires one detecting system for each measuring-control volume to avoid spatial ambiguity and to resolve the velocity differences.
In a publication entitled, "LDV Optical System with Multifrequency Shifting for Simultaneous Measurement of Flow Velocities at Several Points" by N. Nakatani et al, J. Phys. E: Sci. Instr., Vol. 13, pp 172-173, 1980, the need for one detecting system for each measuring-control volume is eliminated by using a rotating radial diffraction grating which will generate several beams with different frequencies. These beams render each of the measuring control volume distinctive such that the scattered light from all of them can be detected by a single photodetector from which the velocity in each volume can be determined. Such a system however can be rather expensive since resolution of signals is limited by the number of grating grooves and the driving motor speed which are both very cost dependent. In addition, for each spatial resolution, a lens of specific focal distance is required, which renders this system inflexible in application.